The present invention relates to a pressure screen. In particular the invention relates to the structure of a pressure screen used in the wood processing industry, providing an improvement in the working reliability and safety of the device.
The pressure screens (as a general example in U.S. Pat. No. 4,634,521) used in the wood processing industry most often comprise a stationary screen drum (cylinder) within a substantially cylindrical outer housing having an inlet for suspension to be screened, and accept and reject outlets. Other inlets and outlets also may be provided, for example, plastics separated in the apparatus, and for dilution or washing liquid to be fed into the apparatus. In most cases, the apparatus is installed in an upright position so that the axis of the substantially cylindrical housing is substantially vertical. Then the top of the apparatus is easy to open or detach from the essentially cylindrical housing and the apparatus may easily be maintained. The shaft of the apparatus passes through the opposite end of the apparatus, i.e. usually the lower end, and usually rotates a rotor, although in some screens the screen cylinder itself is rotatable. Inside the cylindrical housing, there is, in addition to the rotor, also a screen or a sieve cylinder, which in most cases is cylindrical, although a number of cone-shaped screen drums are also used. Usually the screen cylinder is located outside the rotor in the radial direction. The screen cylinder is supported by the housing at both of its ends via intermediate rings. The intermediate rings are secured to the housing of the screen and the screen cylinder is secured to these intermediate rings via a securing flange at both ends.
A screen of another type is disclosed for example in U.S. Pat. No. 5,326,470 which describes many different ways of securing a screen cylinder without compressive loading. This patent discusses several ways of fastening a screen cylinder at its bottom end using a flange extending outwardly from the screen cylinder and an intermediate ring to the outer housing of the screening apparatus and at its top end by a flange extending inwardly from the screen cylinder to a centrally arranged plate sealing the top of the screen cylinder. The rotor of the screen apparatus is disposed outside of the screen cylinder.
Until recently problems had existed relating to the securement of the screen cylinder to the intermediate rings in such a way as to obtain maximum strength. Before the securing methods disclosed in U.S. Pat. No. 5,326,470 were introduced, screen cylinders were secured to intermediate rings in such a way that axial compression stress was directed to the cylinders (see for instance, FIG. 1 of DE-A1-32 40 487). If the screen cylinder was adequately robust and the application stressed the screen cylinder relatively little, no problems arose. On the other hand, as wire screen drums (i.e. wedge wire and bar screen drums) become more common the mechanical endurance of the screen cylinders became essentially weaker so that pulp and paper mills found in particular the compression stress to cause problems which were solved for example by the screen drum securing methods described in the U.S. Pat. No. 5,326,470 in which the screen cylinder is subjected to tensile stress which substantially eliminates the danger of deflection of individual screen bars. However, this patent specially discusses only instances where the rotor is positioned outside the screen cylinder.
Despite the fact that one problem in the securing of a screen cylinder was, at least partially, eliminated a number of new problems were revealed which were previously hidden behind the more serious problems discussed above. It has been found now, however, that it is very difficult to screw securing bolts on the screen cylinder tight enough that they do not come off gradually, for example as a result of screen vibration. When conventional securing methods are used, a small axial clearance is common at the upper end of the screen drum between the screen drum flange and the intermediate ring attached to the housing of the screen which is due to the many tolerances of the manufacturing technique and often even differences in tolerances of different manufacturers. In many cases, the screen drum is not delivered by the same manufacturer who originally manufactured the pressure screen. However, the clearance mentioned allows the bolt securing to "live", for example according to the temperatures or other stresses, so that the securing bolts of the drum are loosened quite easily.
A preferred embodiment of the invention provides a simple and advantageous solution to this problem. An important feature of the approach according to the invention is that there is a conical outer surface of the flange at the bottom end of the screen drum and that it is provided with at least one substantially annular groove, and that a flexible material ring is positioned in that groove.
Another drawback of the securing methods based on the tensile stress of the screen drum described in U.S. Pat. No. 5,326,470 are their complexity. In order to secure the drum in a manner that creates tensile stress, according to the embodiment of that patent illustrated in FIGS. 4 to 9 and 11, the drum must be secured by bolts at its lower end even inside the drum, which means that in practice the rotor must be detached from the pressure screen before the drum can be replaced. In the embodiments of the drawing figures of this patent, only one allows bolting the screen cylinder only at its upper end, and even then the rotor must be removed from its operation position in order to detach the screen cylinder because the inner diameter of the securing flange of the lower end of the screen cylinder is smaller than the outer diameter of the rotor. In other words, in the past utilization of a securing method providing tensile stress presupposed the use of a flange extending inside the screen cylinder which in turn meant that the rotor must be detached for the replacement or maintenance of the screen cylinder.
International patent application PCT/SE94/00013 discloses an approach in which the screen cylinder is mounted in a way so that at both ends of the screen cylinder there is a radial slot and at the slot there is a groove receiving for example an O-ring which seals the slot and at the same time supports the screen cylinder radially in place, i.e. so that the screen cylinder "floats". In the axial direction the screen drum is supported at least at one of its ends by pins extending through the drum to the body of the screen so that the drum cannot move axially or be turned. A problem is, however, that when a drum is left floating in the radial direction only supported by rubber rings, pressure pulses always vibrate the drum to some extent and even a small movement wears out both the sealing members and the pins.
An approach according to another preferred embodiment of the invention eliminates these problems. In other words, according to the invention, the pressure cylinder may be replaced without detaching the rotor, and it supports the cylinder in its place so that pressure pulses cannot move the screen cylinder.
According to one aspect of the present invention a pressure screen is provided comprising the following components: An outer housing. An inlet for material to be screened, and outlets for accepts and rejects, into and from the outer housing. A stationary screen drum disposed inside the outer housing. A rotatable rotor disposed inside the screen drum for rotation with respect thereto. The screen drum having a bottom end with a first flange, and a top end with a second flange. Bottom and top intermediate rings secured to the outer housing, the bottom intermediate ring having an inner edge with a substantially conical surface opening toward the top ring. The second flange connected to the top intermediate ring. The first flange having an outer substantially conical downwardly tapering surface having at least one substantially annular groove with at least one primarily flexible material ring therein. And the first flange outer surface supported by the bottom ring substantially conical surface.
The primarily flexible material ring may be affixed to the bottom intermediate ring substantially conical surface. The second flange may be connected to the top intermediate ring by a plurality of pin bolts cooperating with nuts having a substantially hexagonal shape; and the assembly may further comprise a structure extending above the top intermediate ring which engages the side surfaces of substantially all of the nuts to prevent the nuts and pin bolts from unscrewing. The primarily flexible material ring may include a rigid material stiffening element therein.
According to another aspect of the present invention a pressure screen is provided comprising the following components: An outer housing. An inlet for material to be screened, and outlets for accepts and rejects, into and from the outer housing. A stationary screen drum disposed inside the outer housing. A rotatable rotor disposed inside the screen drum for rotation with respect thereto. The screen drum having a bottom end, and a top end with a flange. Bottom and top intermediate rings secured to the outer housing. The flange connected to the top intermediate ring, and the screen drum bottom end supported by the bottom intermediate ring. And the bottom intermediate ring comprising a substantially conical inner edge surface having at least one annular groove with at least one primarily flexible material ring therein.
In the pressure screen described above, the at least one primarily flexible material ring may include one or more bands or rings of a relatively rigid stiffening material, such as steel bands. The screen drum is preferably a wire drum (that is a wedge wire or a bar screen drum).
According to another aspect of the present invention, a stationary screen drum (for example a wire drum) for use in a pressure screen is provided comprising the following components: A screen drum body having a screening surface, a top end, and a bottom end. A flange provided on the bottom end, the flange having an outer substantially conical surface having a downward taper. And at least one substantially annular groove provided in the flange outer substantially conical surface. At least one substantially annular ring of primarily flexible material may be disposed in said substantially annular groove.
According to still another aspect of the present invention there is provided a method of replacing or maintaining a stationary screen drum within a pressure screen housing including an inlet for material to be screened, outlets for accepts and rejects, and a rotor disposed within and distinct from the screen drum and rotatable about a substantially vertical axis. The method comprises: (a) mounting a screen drum devoid of a bottom interior mounting flange within the outer housing so that the screen drum is subjected primarily to tensile stress, and is not subjected to axial compression stress that adversely affects operation thereof; and (b) when necessary or desired, accessing the screen drum to effect replacement or maintenance thereof without detaching the rotor. The method wherein (a) and (b) are practiced may utilize a wire screen drum as the screen drum. In the method (a) may be practiced utilizing cooperating substantially conical surfaces on the screen drum and on an attachment to the pressure screen housing, at least one of the surfaces having at least one substantially annular groove therein; and providing at least one primarily flexible material ring in the substantially annular groove.
It is the primary object of the present invention to effectively mount a pressure screen drum within a pressure screen housing so that it is not subjected to axial compression stress that adversely affects operation thereof, yet allows ready replacement or maintenance of the screen drum without having to detach the rotor of the pressure screen in which the screen drum is provided. This and other objects of the invention will become clear from an inspection of the detailed description of the invention and from the appended claims.